1. Field of the Invention
The present invention relates to a process for the purification of crude polyorganophosphazenes produced by the cosubstitution of polydichlorophosphazenes.
The polyorganophosphazenes resulting from the cosubstitution reaction of polydichlorophosphazenes by nucleophilic reactants comprise a plurality of recurring units of the formula: ##STR1## in which the symbols R, which have at least two different definitions within a single recurring unit and/or from one unit to another, are particularly alkoxy, fluoroalkoxy, phenoxy, alkylamino or arylamino groups. Examples of such polyorganophosphazenes are given, e.g., in Polymer News, volume 5, No. 1, pages 9-17 (1978).
Among such polyorganophosphazenes, this invention more especially relates to the amorphous polymers, which occur generally in the form of a rubbery mass having more or less elastomeric properties.
2. Description of the Prior Art
It is known to this art that the isolation of the subject polyorganophosphazenes from the medium in which they were synthesized presents certain difficulties. Indeed, the last stage in their preparation entails reacting nucleophilic compounds such as sodium alcoholates or sodium phenates with polydichlorohosphazene, usually in a hydrocarbon or an ether, or a mixture of the two (see, for example, U.S. Pat. Nos. 4,514,550, 3,970,533, 4,576,806 or the article in Polymer News, loc. cit.).
Upon completion of the reaction, the excess of nucleophile is generally neutralized by means of an acid, such as hydrochloric acid or sulfuric acid.
The polyorganophosphazene is thus present in a concentration generally ranging from 5 to 35% in a medium containing a hydrocarbon and/or an ether, an excess of phenol or alcohol, possibly other substitution reactants, and a salt, generally of sodium.
It has already been proposed to the art (U.S. Pat. No. 4,116,785) to precipitate the polymer by contacting the solution described above with a nonsolvent such as methanol. Such a process requires the use of a quantity of solvent which may constitute 20 to 40 times the quantity of polymer to be separated. Hence, to obtain the polymer requires the use of a reactor of large capacity in relation to the quantity of polymer recovered. Moreover, when the polymer occurs in the form of a rubbery mass, which is more or less tacky, such as is produced when the polymer bears at least two different substituents, the extraction of the polymer from the reactor in which it was produced must typically be done manually.